*will* they? Probably not, at least at first. I occasionally tether a device (laptop or tablet) to my N1 and have never seen any sign that AT&T knows or cares. *can* they? Yup. They are well within their rights to add the tethering plan to your contract if they determine you are doing it. I suppose it depends on what is the usage threshold for passing under the radar.

I'd say the best thing to do is, since you're doing something that AT&T doesn't want you to do is to keep your use in check and stay under their radar. Don't use it unless absolutely necessary and when you're done, turn it off.